This invention relates to the manufacture of flexible plastic film handling such as polyethylene film, which may be used in bags, sheets, etc.
Polyethylene film is typically created using a tube extrusion process, in which a tube of the softened plastic material is extruded through an annular die, and is then inflated and expanded until the tube wall is a desired thickness and layflat dimension. The film is typically made in a range of about 0.3 mils to several mils, depending on the end use. After the tube of material is extruded, it is drawn vertically upward, typically a distance of forty feet or higher. The air within the tube expands the tube outward until the plastic sets. On the journey upward, the extruded tube has to be flattened so that it may pass between nip rollers, and then passed to a reel or mandrel where the film is rolled and can be stored for further processing. The flattening is accomplished by a collapsing frame, that is, two facing arrangements of collapsing boards that urge the tubular extrusion to a flattened state by the time the tube reaches the nip rollers. These are usually configured as collapsing frames, each of which has a number of collapsing boards that extend parallel and one above the other on the frame. The nip rollers then press the tube and create a lay-flat film of two thicknesses.
In some cases, it is desired to fold the ends of the extruded tube in prior to the tube reaching the nip rollers, so that the resulting flattened tube is four layers, i.e., four thicknesses of film. To this end gusset boards, also called gusseters or plows, are positioned between the collapsing boards to form inward folds, or gussets, on opposite sides of the extruded tube as the tube progresses upwards. These gusset boards are typically triangular in shape with the apex at the upper end, and with side surfaces that extend back away from the axis to keep the film flat and in place as it passes over the gusseter. The side surfaces have their the greatest lateral dimension nearer the apex, where the folds or gussets are the deepest. The extrusion arrangement invariably has the nip rollers and roll-up equipment positioned on a floor or mezzanine above the top of the collapsing frames, with a round or octagonal opening through which the tube of film passes. When gusseters are used, the tips or apexes of the gusseter boards project upwards through the opening in the mezzanine This requirement means that the mezzanine structure itself obstructs the top of the gusseter if it is desired to move the gusseter out of the way when the equipment is changed over for producing un-gusseted film.
Current techniques for changing over from gusseted film production to ungussetted film production require dismantling or removing the gusseter boards from the extrusion equipment because the nose of the gusseter collides with the frame or mezzanine when the gusseter is tilted back away from the extrusion axis. This requires the labor of several workers for times of an hour or more. There have been proposals for the nose or tip of the gusseter to be removable from the remainder of the gusset board, but this also requires time and effort to remove the nose and later re-install it.